1. Field of Invention
This invention relates to an improved sorbent for removing NOx and SOx from exhaust gases and the improved removal process using that sorbent.
2. Description of the Previously Published Art
The NOXSO process, described in U.S. Pat. No. 4,798,711, uses a porous alumina sorbent containing sodium to adsorb NOx and SOx from flue gases. The process involves using beads of a sufficient size so that they can be used in a fluidized bed system. When discussing the attrition test they use beads of 10 to 20 mesh. Since a 20 mesh screen has openings of about 840 microns, the particles will be at least 840 microns in diameter. After the adsorption the nitrogen- and sulfur-laden sorbent is regenerated by heating the sorbent in a reducing atmosphere.
In addition to other preferred reducing atmospheres for regenerating, methane gas is also disclosed. However, by using natural gas or methane the regenerator operates at a higher temperature of about 650.degree. C. which tends to further reduce the surface area of the alumina beads.
As this NOXSO sorbent cycles back and forth from adsorption to regeneration, it undergoes transformations which reduce its surface area. In the fresh condition the alumina sorbent has a surface area on the order of about 200 m.sup.2 /g. After about 100 cycles the surface area of the sorbent is reduced to below 50 m.sup.2 /g. When the surface area becomes that low, the performance becomes unacceptable.
In U.S. Pat. No. 5,180,703, an improved sorbent is made by adding a stabilizer to an alumina forming slurry. The preferred alumina stabilizer is silica which can be added in the form of sodium silicate. The resulting silica alumina coprecipitate is preferably filtered, washed and dried to a powder. The powder is slurried in a water/nitric acid/acetic acid mixture and dripped in a drip column containing a NH.sub.3 -liquid hydrocarbon phase on top of an aqueous ammonia phase. The silica stabilized alumina beads are dried and calcined and then impregnated with up to 1-20 wt % with an active ingredient of either an alkali metal or alkaline earth metal and preferably with sodium at about 4-6 wt %, by the incipient wetness technique. The beads are screened to pass through 10 mesh and remain on 20 mesh. Since a 20 mesh screen has openings of 840 microns, the particles will be at least 840 microns in diameter.
3. Need for the Invention
The beads have sodium carbonate on them. When this strong base contacts the acidic SO.sub.2 present in flue gas, a very rapid acid-base reaction occurs. Initially the SO.sub.2 and NO gas reacts with the sodium on the outer surface of the bead. Soon the outer surface sites are covered and then the gas must diffuse inwardly to find fresh sodium sites for reaction. This need for diffusion prior to adsorption will slow down the performance of the sorbent over time.
In order to improve the performances of the sorbent, one can keep the mass transport resistance by diffusion of the process to a minimum by employing smaller and smaller sized particles. The use of small sorbent particles no longer necessitates the deep penetration of the flue gas components into the porous structure of the sorbent by diffusion. This has the advantage that a higher removal performance can be sustained over a longer contact period on an equal sorbent weight basis than with larger sized beads. Smaller sorbent also has the advantage of a substantially higher external geometric area for the same weight of sorbent than with larger beads which contributes to the higher removal performance. Since the smaller sized sorbent has a considerably smaller entrainment velocity, it is ideally suited in a riser type reactor because it is easily carried along with the flue gas under normal process conditions.
4. Objects of the Invention
It is an object of this invention to develop a small size sorbent with a stable surface area and pore volume at process conditions in the presence of sodium or other alkali metals for use in a gas-solid contacting apparatus.
It is further object of this invention to provide a small sorbent particle having a particle size of from about 10 to 500 microns for use in adsorbing NOx and SOx from a flue gas.
It is further object of this invention to provide a stabilized surface area sorbent having a particle size smaller than about 500 microns for use in adsorbing NOx and SOx from a flue gas.
It is further object of this invention to provide a sorbent with stable surface area under hydrothermal conditions in the presence of sodium.
It is further object of this invention to provide a sorbent with low attrition characteristics.
It is further object of this invention to provide alternative processes to produce a stabilized surface area sorbent for use in adsorbing NOx and SOx from a flue gas.
It is further object of this invention to provide a process for the continued removal of nitrogen oxides and sulfur oxides from a gas containing them with a stabilized sorbent which can undergo regeneration at high temperatures and under hydrothermal conditions.
These and further objects will become apparent as the description of the invention proceeds.